1. Field of the Invention
This invention relates to a vehicle steering control system capable of controlling vehicle steering response characteristics freely.
2. Description of the Prior Art
In a conventional vehicle having only a mechanical steering linkage, front wheels are steered in accordance with a steering input such as a steering wheel angular displacement. Therefore, the steering response characteristics are fixedly determined by the make of the vehicle.
3. Description of the Related Art
New steering control systems are disclosed in the following four copending U.S. patent applications in the name of Ken ITO et al. (1) Ser. No. 755,352 filed July 16, 1985; (2) Ser. No. 773,650 filed Sept. 9, 1985; (3) Ser. No. 773,621, filed Sept. 9, 1985 (4) Ser. No. 773,622, filed Sept. 9, 1985. Each of these control systems postulates a desired vehicle having desired cornering characteristics independently of the actual vehicle controlled by the system. By using data (vehicle parameters) of the desired vehicle, and equations of motion of the desired vehicle, a control unit such as a microcomputer of each system determines a desired value of a vehicle motion variable such as yaw rate corresponding to instantaneous values of a steering wheel ang1e and vehicle speed of the controlled vehicle. That is, desired value of the motion variable is determined according to the desired characteristics of the desired vehicle independent of the actual characteristics of the controlled vehicle. Then. the control system steers either or both of front and rear wheel pairs of the controlled vehicle to such an amount that the actual vehicle motion variable becomes equal to the desired value.
Therefore, these control system can control the cornering characteristics of the controlled vehicle freely without the restraint of the actual body construction of the controlled vehicle. For example, these control systems can endow the controlled vehicle of a sedan type with characteristics of a sports car by choosing the desired vehicle of a sports car type.
These systems can control only one motion variable when only one of the front and rear wheel pairs is manipulated. In the case that the yaw rate is chosen as a controlled variable, for example, these systems cannot provide satisfactory characteristics of a vehicle lateral acceleration especially when the steering wheel is turned rapidly at high vehicle speeds.